Shella_A , just to be clear, the manual import process allows a blank value for the "items per selling unit", and this is a significant bug, NOT a quirk. (The core program does not allow it). I raised this with product management over a year ago, but unfortunately it has been deemed a low priority.
It's behind, or related to, the 4077 import error which crops up fairly regularly. If the drain on support staff time, and the impact on users, was considered, perhaps the priority level would be raised? There ought to be an allocation of developer time to address important issues that may never get ranked highly enough on their own.